U.S. patent application number 08/312295 was filed with the patent office on 2002-03-14 for biaxially oriented polypropylene film structure having improved mechanical and barrier properties.
Invention is credited to DRIES, THOMAS, MURSCHALL, URSULA, PEIFFER, HERBERT, SCHLOEGL, GUNTER.
Application Number | 20020032295 08/312295 |
Document ID | / |
Family ID | 25929932 |
Filed Date | 2002-03-14 |
United States Patent
Application |
20020032295 |
Kind Code |
A1 |
PEIFFER, HERBERT ; et
al. |
March 14, 2002 |
BIAXIALLY ORIENTED POLYPROPYLENE FILM STRUCTURE HAVING IMPROVED
MECHANICAL AND BARRIER PROPERTIES
Abstract
A single-ply or laminar biaxially oriented polypropylene film
structure is described. The n-hexane-insoluble fraction of the film
has a chain isotaxy index, measured by means of .sup.13C-NMR
spectroscopy, of at least 95%. The base ply of the laminar
structure (or the single ply of the single-ply structure) contains
essentially no hydrocarbon resin or low molecular weight resin. The
modulus of elasticity of the film structure in the longitudinal
direction is greater than 2,500 N/mm.sup.2. The modulus of
elasticity of the film structure in the transverse direction is
greater than 4,00 N/mm.sup.2. A process for the production of the
polypropylene film structure and the use thereof are also
described.
Inventors: |
PEIFFER, HERBERT; (MAINZ,
DE) ; DRIES, THOMAS; (SCHWABENHEIM, DE) ;
MURSCHALL, URSULA; (NIERSTEIN, DE) ; SCHLOEGL,
GUNTER; (KELKHEIM, DE) |
Correspondence
Address: |
CONNOLLY AND HUTZ
P.O. BOX 2207
WILMINGTON
DE
19899
|
Family ID: |
25929932 |
Appl. No.: |
08/312295 |
Filed: |
September 26, 1994 |
Current U.S.
Class: |
526/348.1 ;
428/212; 428/215; 428/216; 428/220; 428/327; 428/516;
525/333.8 |
Current CPC
Class: |
C08L 23/142 20130101;
Y10T 428/31913 20150401; B32B 2250/242 20130101; C08L 23/142
20130101; Y10T 428/24967 20150115; B32B 2307/518 20130101; B32B
27/20 20130101; C08J 2323/10 20130101; C08L 23/12 20130101; Y10T
428/24942 20150115; B32B 27/32 20130101; C08L 2205/03 20130101;
C08J 5/18 20130101; Y10T 428/254 20150115; B32B 2309/105 20130101;
C08L 83/04 20130101; B32B 27/08 20130101; C08L 23/142 20130101;
C08L 23/06 20130101; H01G 4/18 20130101; Y10T 428/24975 20150115;
C08L 2666/02 20130101; C08L 83/00 20130101 |
Class at
Publication: |
526/348.1 ;
525/333.8; 428/212; 428/215; 428/216; 428/220; 428/327;
428/516 |
International
Class: |
C08F 210/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 27, 1993 |
DE |
P 43 32 835.0 |
Nov 2, 1993 |
DE |
P 43 37 250.3 |
Claims
What is claimed is:
1. A biaxially oriented polypropylene film structure comprising at
least one propylene polymer-containing ply, said film structure
having: an n-heptane-insoluble fraction which has a chain isotaxy
index, measured by means of .sup.13C-NMR spectroscopy, of at least
95%, a modulus of elasticity of the film structure in the
longitudinal direction which is greater than 2,500 N/mm.sup.2 and a
modulus of elasticity of the film structure in the transverse
direction which is greater than 4,000 N/mm.sup.2, said propylene
polymer-containing ply being essentially free of hydrocarbon resin
having a weight average of the molecular weight average M.sub.w
less than 5,000.
2. The polypropylene film structure as claimed in claim 1, wherein
said film structure comprises a plurality of plies, and said
propylene polymer-containing ply is the base ply of said film
structure.
3. The polypropylene film structure as claimed in claim 1, wherein
the water vapor transmission, WVT, of the film structure, as
defined in accordance with German Engineering Standard DIN 53 122
obeys the formula 3 WVT C d where d is the thickness of the film in
.mu.m and C is 22.5 g.multidot..mu.m/m.sup.2.
4. The polypropylene film structure as claimed in claim 1, wherein
the modulus of elasticity of the film structure in the longitudinal
direction is greater than 2,700 N/mm.sup.2 and the modulus of
elasticity of the film structure in the transverse direction is
greater than 4,200 N/mm.sup.2.
5. The polypropylene film structure as claimed in claim 1, wherein
the tensile strength of said film structure in the longitudinal
direction is greater than 160 N/mm.sup.2 and that in the transverse
direction is greater than 230 N/mm.sup.2, and the elongation at
break in the longitudinal direction is less than 140% and that in
the transverse direction is greater than 60%.
6. The polypropylene film structure as claimed in claim 2, wherein
said base ply contains a polypropylene whose n-heptane-insoluble
fraction has a chain isotaxy index, measured by means of
.sup.13C-NMR spectroscopy, of at least 95%.
7. The polypropylene film structure as claimed in claim 2, wherein
said base ply contains a polypropylene whose M.sub.w/M.sub.n is
greater than 6.
8. The polypropylene film structure as claimed in claim 2, wherein
the propylene polymer of said base ply has been degraded by a
peroxide mechanism and the M.sub.w/M.sub.n of said propylene
polymer is less than 6.
9. The polypropylene film structure as claimed in claim 8, wherein
the degradation factor resulting from degradation by a said
peroxide mechanism is 3 to 15.
10. The polypropylene film structure as claimed in claim 2, which
has a top ply comprising .alpha.-olefinic polymers on at least one
side.
11. The polypropylene film structure as claimed in claim 10,
wherein the top ply or plies contains or contain a propylene
homopolymer whose n-heptane-insoluble fraction has a chain isotaxy
index, measured by means of .sup.13C-NMR spectroscopy, of at least
95%.
12. The polypropylene film structure as claimed in claim 10,
wherein the polymer of the top ply or plies is degraded by a
peroxide mechanism and the degradation factor is in the range from
3 to 15.
13. The polypropylene film structure as claimed in claim 2,
comprising a said base ply and at least one top ply, wherein
interlayer or interlayers on one or both sides and comprising
.alpha.-olefinic polymers is or are applied between the base ply
and the top ply or plies.
14. The polypropylene film structure as claimed in claim 2,
comprising a said base ply and at least one interlayer, wherein the
interlayer or interlayers contains or contain a propylene
homopolymer whose n-heptane-insoluble fraction has a chain isotaxy
index, measured by means of .sup.13C-NMR spectroscopy, of at least
95%.
15. The polypropylene film structure as claimed in claim 2,
comprising a said base ply and at least one interlayer, wherein a
polymer of the interlayer or interlayers has been degraded by a
peroxide mechanism, and the degradation factor is in the range from
3 to 15.
16. The polypropylene film structure as claimed in claim 2,
comprising a said base ply and at least one interlayer, wherein a
said interlayer contains a pigment.
17. The polypropylene film structure as claimed in claim 2, wherein
said base ply contains an antistatic agent.
18. The polypropylene film structure as claimed in claim 2, wherein
said base ply contains a pigment.
19. The polypropylene film structure as claimed in claim 2,
comprising a said base ply and at least one top ply, wherein a said
top ply contains a lubricant and an antiblocking agent.
20. The polypropylene film structure as claimed in claim 2,
comprising a said base ply and at least one interlayer, wherein a
said interlayer contains a neutralizing agent, a stabilizer, an
antistatic agent an anti-blocking agent, or a combination
thereof.
21. The polypropylene film structure as claimed in claim 2,
comprising a said base ply and at least one top ply, wherein a said
top ply is sealable.
22. The polypropylene film structure as claimed in claim 2,
comprising a said base ply and at least one top ply, wherein a said
top ply is not sealable.
23. The polypropylene film structure as claimed in claim 2, wherein
the thickness of the film structure ranges from 4 to 60 .mu.m, said
base ply accounting for about 40 to 60% of the total thickness.
24. The polypropylene film structure as claimed in claim 1, wherein
said film structure is a single-ply film.
25. A process for the production of a polypropylene film structure
as claimed in claim 1, wherein the orientation in the longitudinal
direction is effected with a longitudinal stretching ratio of 5:1
to 9:1 and that in the transverse direction is effected with a
transverse stretching ratio of 5:1 to 9:1.
26. A method of packaging comprising the step of packaging the
material or object to be packaged in a packaging film comprising
the film structure as claimed in claim 1.
27. An enclosed package comprising a polypropylene film structure
as claimed in claim 1.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a polypropylene film or film
structure (single-ply or laminar) having improved barrier
properties with regard to the passage of water vapor and oxygen and
improved mechanical properties.
DESCRIPTION OF THE PRIOR ART
[0002] The improvement in the mechanical properties of films, in
particular of films for the packaging sector, has recently become
more important. For cost reasons and environmental reasons, the
packaging industry requires increasingly thin films with unchanged
running characteristics in machines and unchanged or improved
barrier properties, in particular with regard to the passage of
water vapor and oxygen.
[0003] However, thinner films have disproportionately poorer
rigidity in the machine direction and hence substantially poorer
running characteristics in machines, in particular in today's
high-speed wrapping machines. Furthermore, the barrier properties,
too, decline disproportionately with the reduction in the film
thickness. Owing to the poorer barrier properties of thin films,
the protective effect of the film to prevent spoiling of the
contents is greatly limited.
[0004] The rigidity (R) of the film is proportional to the modulus
of elasticity (E) and to the third power of the thickness (d)
[R=E.multidot.d.sup.3]. In the case of relatively thin films, it is
therefore only possible to compensate the loss of rigidity via the
modulus of elasticity of the film.
[0005] Increasing the modulus of elasticity (E modulus) in the
machine direction has long been the subject of intensive efforts,
because this mechanical property is directly related to the
suitability for use and hence directly determines the processing
behavior.
[0006] As described in the product surveys from the companies Mobil
Plastics Europe and Hoechst AG, for example, the tensile modulus of
elasticity (DIN 53 457, ASTM 882) of conventional boPP films in the
longitudinal direction is between 2,000 and 2,200 N/mm.sup.2,
regardless of the thickness.
[0007] The water vapor transmission (WVT) and oxygen transmission
(OT) of boPP films decrease with increasing film thickness. In the
conventional thickness range of boPP films (4 to 100 .mu.m), there
is, for example, approximately a hyperbolic relationship
(WVT.multidot.d=const.) between the water vapor transmission (WVT)
and the thickness (d). The constant essentially depends on the raw
material composition and the stretching conditions. For boPP
packaging films according to the prior art, the constant has a
value of about: const.=28 g.multidot..mu.m/m.sup.2.multido- t.d.
The water vapor transmission was measured here according to DIN 53
122. The stated product surveys reveal that, for example, the water
vapor transmission of a 25 .mu.m thick boPP film is 1.1 g/M.sup.2
d.
[0008] It is known that, in the case of boPP films, the modulus of
elasticity in the machine direction can be increased either by
means of process engineering or by means of raw material
modifications or a combination of the two possibilities.
[0009] A possible method for the production of high-strength
polypropylene films is a three-stage or multistage stretching
process, as described, for example, in EP-B-0 116 457. However,
such a production process has the disadvantage that it requires an
additional apparatus for subsequent longitudinal stretching and is
therefore very expensive. Moreover, it is very susceptible to
breakdowns in the course of production, for example tears in the
film.
[0010] Furthermore, such subsequently longitudinally stretched
films exhibit longitudinal shrinkage which is substantially higher
compared with only biaxially stretched films and which as a rule
prevents the films from withstanding thermal drying, as is still
usual in some cases, for example after application of adhesive
materials, without undesirable shrink folds.
[0011] The modification of the raw materials used for the
production of high-strength polypropylene films with various
hydrocarbon resins is described, for example, in U.S. Pat. No.
3,937,762. Such modification of raw materials permits the
production of polypropylene films whose mechanical strength in the
longitudinal direction is substantially improved compared with
films of unmodified raw materials but does not reach the values of
subsequently longitudinally stretched films, and shrinkage in the
longitudinal direction is likewise relatively high.
[0012] EP-A-0 406 642 describes a boPP film having high mechanical
strength. The high modulus of elasticity in the longitudinal
direction is achieved if the base ply contains 5 to 30% by weight
of a hydrocarbon resin and 0.01 to 1.0% by weight of a nucleating
agent. This patent publication provides no information about
barrier properties. In the Examples, a resin concentration of 20%
by weight is mentioned.
[0013] Such high resin concentrations lead to problems in film
production. In particular, deposits occur after a short time on the
screw of the plasticating extruder and on the rolls of the
longitudinal stretching unit. Furthermore, the addition of
nucleating agents in the stated concentration leads to optical film
defects in the form of so-called "specks" and "bubbles", which of
course are extremely undesirable. In addition, the regenerated
material from such films can no longer be used owing to the
tendency to agglomerate in the film production process.
Furthermore, the stretching ratios stated in Examples 3 to 6 of
EP-A-0 406 642 cannot be realized on any production machine at the
conventional speeds with the homopolymer described there. Film
constantly tears, particularly during transverse stretching.
[0014] Outstanding mechanical properties can be achieved by the
combination of the addition of a resin to the raw material used
with a subsequent longitudinal stretching process. A corresponding
procedure is described in EP-A-0 079 520; moduli of elasticity in
the longitudinal direction of 4,000 to 6,000 N/mm.sup.2 are
achieved. However, this process, too, has the disadvantage that an
expensive subsequent longitudinal stretching process susceptible to
faults is required.
[0015] U.S. Pat. No. 4,921,749 (an apparent counterpart of EP-A-0
247 898) describes a sealable boPP film having improved mechanical
and optical properties. The sealability of the film and the water
vapor and oxygen transmission are also improved. All improvements
result from the addition of a low molecular weight resin to the
base ply. The amount of resin is between 3 and 30% by weight. The
resin has a molecular weight of substantially less than 5,000,
preferably less than 1,000, and is, for example, 600. The softening
point of the resin is 120 to 140.degree. C.
[0016] EP-A-0 468 333 describes a sealable film having improved
barrier properties with regard to the passage of water vapor and
oxygen in combination with good sliding properties and low
shrinkage values. The characterizing features of this boPP film are
that it is composed of a base ply which comprises a polypropylene
and a hydrocarbon resin having a softening point greater than
140.degree. C., and that it has at least one sealable top, ply
which, if required, additionally contains a hydrocarbon resin. The
base ply and the top ply contain at least one anti-blocking agent
and/or one lubricant.
[0017] In U.S. Pat. No. 4,921,749 and EP-A-0 468 333, high
concentrations of hydrocarbon resin are used in order to enhance
the barrier properties. Such high resin contents lead to problems
in film production. In particular, resin deposits occur on the
screw of the extruder and on the longitudinal stretching rolls
after a short time. Owing to the high resin contents, the films
have high high-temperature blocking values and exhibit a
troublesome tendency to block during further processing.
[0018] A principal objective of the present invention was to
provide a biaxially oriented polypropylene film which is
distinguished by a high modulus of elasticity in the machine
direction and enhanced barrier properties with regard to the
passage of water vapor and oxygen. The disadvantages of the
subsequent longitudinal stretching process, such as technical
conversions in the production machine, breakdowns due to frequent
tearing of the film and high residual shrinkage of the boPP films,
are to be avoided. Furthermore, it must be ensured that the
regenerated material can be added again in a concentration of 20 to
50% by weight, based on the total weight of the film. The film must
be capable of being produced so that it runs reliably and
withstands the process at production speeds of up to 400 m/min
without resin deposits occurring in the preparation process. Other
physical film properties which are required with regard to the use
thereof as packaging film and/or as laminating film must not be
adversely affected. The film should have a high gloss, no optical
defects in the form of specks or bubbles, good scratch resistance,
trouble-free running with low film thickness on high-speed
packaging machines and, in the case of transparent film types,
little opacity of the film.
SUMMARY OF THE INVENTION
[0019] The above-described objective can be achieved by a biaxially
oriented polypropylene film structure (single-ply or laminar) whose
characterizing features are that the n-heptane-insoluble fraction
of the film structure has a chain isotaxy index, measured by means
of .sup.13C-NMR spectroscopy, of at least 95% and that the base ply
(in the case of a film structure having a plurality of plies) or
the single ply contains essentially no hydrocarbon resin and that
the modulus of elasticity of the film in the longitudinal direction
is greater than 2,500 N/mm.sup.2 and the modulus of elasticity of
the film in the transverse direction is greater than 4,000
N/mm.sup.2. Thus, the base ply (or the entire film structure, in
the case of a single-ply film) is essentially free of hydrocarbon
resins (whether of high softening point or low softening point)
having a weight average of the molecular weight average M of
substantially less than 5,000. Hydrocarbon resins are defined to be
a nonhydrogenated styrene polymer, a methylstyrene/styrene
copolymer, a pentadiene or cyclopentadiene copolymer, an .alpha.-
or .beta.-pinene polymer, resin or rosin derivatives or terpene
polymers and hydrogenated compounds thereof or a hydrogenated
.alpha.-methylstyrene/vi- nyltoluene copolymer or a mixture
thereof.
[0020] According to the invention, the film structure can be
single-ply and is then composed only of the base ply described
below (in this case "base ply" is synonymous with single ply) . The
base ply is defined to be that ply of the film that provides the
greatest thickness. Generally the base ply is 40%, preferably 50 to
98%,, of the overall film thickness. In a preferred embodiment, the
film has, on its base ply, at least one top ply or if required top
plies on both sides. In a further embodiment, the film has on its
base ply at least one interlayer or if required interlayers on both
sides.
BRIEF DESCRIPTION OF THE DRAWING
[0021] The sole Figure of the Drawing, FIG. 1, is a schematically
enlarged representation of a .sup.13C-NMR spectrum of an
ethylene/propylene copolymer, a raw material useful in making film
structures of this invention.
DETAILED DESCRIPTION
[0022] The base ply of a film structure of this invention (or the
single ply of the single-ply embodiment of the invention) contains
in general at least 85% by weight, preferably 85 to 100% by weight,
in particular 90 to 100% by weight, based in each case on the base
or single ply (as the case may be), of a propylene homopolymer
described below.
[0023] This propylene homopolymer contains at least 90% by weight,
preferably 94 to 100% by weight, in particular 98 to 100% by
weight, of propylene. The corresponding comonomer content of not
more than 10% by weight or 0 to 6% by weight or 0 to 2% by weight,
respectively, comprises, if present, in general ethylene. The data
in % by weight are is based in each case on the propylene
homopolymer.
[0024] The propylene homopolymer of the base ply has a melting
point of 140 to 165.degree. C., preferably 155 to 162.degree. C.,
and a melt flow index (measured according to DIN 53 735 under a
load of 21.6N and at 230.degree. C.) of 1.0 to 10 g/10 min,
preferably 1.5 to 6 g/10 min. The n-heptane-soluble fraction of the
polymer is in general 1 to 10% by weight, based on the starting
polymer. The n-heptane-insoluble fraction of the propylene
homopolymer is highly isotactic. The chain isotaxy index of the
n-heptane-insoluble fraction, determined by means of .sup.13C-NMR
spectroscopy, is at least 95%, preferably 96 to 99%.
[0025] The molecular weight distribution of the propylene
homopolymer can vary within wide limits, depending on the field of
use. The ratio of the weight average molecular weight M.sub.H to
the number average molecular weight M.sub.n is in general between 2
and 15.
[0026] In a preferred embodiment of the film according to the
invention, the ratio of the weight average molecular weight M.sub.H
to the number average molecular weight M.sub.n is 2 to 6, very
particularly preferably 3.5 to 5. Such a narrow molecular weight
distribution of the propylene homopolymer of the base ply is
achieved, for example, by peroxidic degradation thereof.
[0027] A measure of the degree of degradation of the polymer is the
so-called degradation factor A, which indicates the relative change
in the melt flow index according to DIN 53 735 of the
polypropylene, relative to the starting polymer. 1 A = MFI 2 MFI
1
[0028] MFI.sub.1=Melt flow index of the propylene polymer before
the addition of the organic peroxide
[0029] MFI.sub.2=Melt flow index of the propylene polymer degraded
by a peroxide mechanism.
[0030] In general, the degradation factor A of the propylene
polymer used is in the range from 3 to 15, preferably 6 to 10.
[0031] Dialkyl peroxides are particularly preferred as organic
peroxides, an alkyl radical being understood as meaning the usual
saturated straight-chain or branched lower alkyl radicals having up
to six carbon atoms. 2,5-Dimethyl-2,5-di-(tert-butylperoxy)-hexane
or di-tert-butyl peroxide are particularly preferred.
[0032] Furthermore, the base ply can, if desired, additionally
contain conventional additives, such as antiblocking agents,
neutralizing agents, stabilizers, antistatic agents, lubricants and
pigments, each in effective amounts. It contains, however,
essentially no resin. The term "essentially" means that a low resin
content which does not influence the film properties is possible,
this content being generally below 1% by weight, based on the total
weight of the film.
[0033] Preferred antistatic agents are alkali metal
alkane-sulfonates, polyether-modified, i.e. ethoxylated and/or
propoxylated polydiorganosiloxanes (polydialkylsiloxanes,
polyalkylphenylsiloxanes and the like) and/or the essentially
straight-chain and saturated aliphatic, tertiary amines which have
an aliphatic radical having 10 to 20 carbon atoms and are
substituted by .omega.-hydroxy-(C.sub.1-C.sub.4)-alkyl groups,
N,N-bis-(2-hydroxyethyl)-alkylamines having 10 to 20 carbon atoms,
preferably 12 to 18 carbon atoms, in the alkyl radical being
particularly suitable. The effective amount of antistatic agent is
in the range from 0.05 to 0.5% by weight. Furthermore, glyceryl
monostearate is preferably used as an antistatic agent, in an
amount of 0.03% to 0.5%.
[0034] Suitable antiblocking agents are inorganic additives, such
as silica, calcium carbonate, magnesium silicate, aluminum
silicate, calcium phosphate and the like, and/or incompatible
organic polymers, such as polyamides, polyesters, polycarbonates
and the like, preferably benzoguanamine/formaldehyde polymers,
silica and calcium carbonate. The effective amount of antiblocking
agent is in the range from 0.1 to 2% by weight, preferably 0.1 to
0.8% by weight. The mean particle size is between 1 and 6 .mu.m, in
particular 2 and 5 .mu.m, particles having a spherical shape, as
described in EP-A-0 236 945 and DE-A-38 01 535, being particularly
suitable.
[0035] Lubricants are higher aliphatic amides, higher aliphatic
esters, waxes and metal soaps as well as polydimethylsiloxanes. The
effective amount of lubricant is in the range from 0.01 to 3% by
weight, preferably 0.02 to 1% by weight. The addition of higher
aliphatic amides in the range from 0.01 to 0.25% by weight to the
base ply is particularly suitable. A particularly suitable
aliphatic amide is erucamide.
[0036] The addition of polydimethylsiloxanes in the range from 0.02
to 2.0% by weight is preferred, in particular polydimethylsiloxanes
having a viscosity from 5,000 to 1,000,000 mm.sup.2/s.
[0037] The stabilizers used can be the conventional compounds
having a stabilizing action for ethylene polymers, propylene
polymers and other a-olefin polymers. The added amount thereof is
between 0.05 and 2% by weight. Phenolic stabilizers, alkali metal
stearates/alkaline earth metal stearates and/or alkali metal
carbonates/alkaline earth metal carbonates are particularly
suitable.
[0038] Phenolic stabilizers in an amount from 0.1 to 0.6% by
weight, in particular 0.15 to 0.3% by weight, and with a molecular
mass of more than 500 g/mol are preferred. Pentaerythrityl
tetrakis-3-(3,5-di-tertiary-buty- l-4-hydroxyphenyl)-propionate or
1,3,5-trimethyl-2,4,6-tris(3,5-di-tertiar-
y-butyl-4-hydroxybenzyl)benzene are particularly advantageous.
[0039] Neutralizing agents are preferably dihydrotalcite, calcium
stearate and/or calcium carbonate having a mean particle size of at
most 0.7 .mu.m, an absolute particle size of less than 10 Am and a
specific surface area of at least 40 m.sup.2/g.
[0040] Pigments comprise those particles which essentially do not
lead to vacuole formation during stretching. The coloring effect of
the pigments is caused by the particles themselves. The term
"pigment" is in general associated with a particle size of 0.01 to
at most 1 .mu.m and thus covers both so-called "white pigments",
which color the films white, and "colored pigments", which impart a
color to the film or render it black. In general, the mean particle
diameter of the pigments is in the range from 0.01 to 1 .mu.m,
preferably 0.01 to 0.5 .mu.m.
[0041] Conventional pigments are materials such as, for example,
alumina, aluminum sulfate, barium sulfate, calcium carbonate,
magnesium carbonate, silicates, such as aluminum silicate (kaolin
clay) and magnesium silicate (talc), silica and titanium dioxide,
among which calcium carbonate, silica and titanium dioxide are
preferably used.
[0042] The base ply contains pigments in general in an amount of 1
to 25% by weight, in particular 2 to 20% by weight, preferably 5 to
15% by weight, based in each case on the base ply. Preferred
pigments are white pigments, in particular TiO.sub.2, silica and
BaSO.sub.4. These pigments preferably have a mean particle diameter
of 0.01 to 0.7 .mu.m, in particular 0.01 to 0.4 .mu.m.
[0043] The titanium dioxide particles comprise at least 95% by
weight of rutile and are preferably used with a coating of
inorganic oxides, as is usually employed as a coating for TiO.sub.2
white pigment in papers or coating materials for improving the
lightfastness. The particularly suitable inorganic oxides include
the oxides of aluminum, silicon, zinc or magnesium or mixtures of
two or more of these compounds. They are precipitated from
water-soluble compounds, for example alkali metal aluminate, in
particular sodium aluminate, aluminum hydroxide, aluminum sulfate,
aluminum nitrate, sodium silicate or silica, in aqueous suspension.
TiO.sub.2 particles having a coating are described, for example, in
EP-A-0 078 633 and EP-A-0 044 515.
[0044] The coating also contains, if required, organic compounds
having polar and nonpolar groups. Preferred organic compounds are
alkanols and fatty acids having 8 to 30 carbon atoms in the alkyl
group, in particular fatty acids and primary n-alkanols having 12
to 24 carbon atoms, as well as polydiorganosiloxanes and/or
polyorganohydrogensiloxanes, such as polydimethylsiloxane and
polymethylhydrogensiloxane.
[0045] The coating on the TiO.sub.2 particles usually comprises 1
to 12 g, in particular 2 to 6 g, of inorganic oxides; if necessary,
0.5 to 3 g, in particular 0.7 to 1.5 g, of organic compounds, based
in each case on 100 g of TiO.sub.2 particles, are additionally
present. It has proven particularly advantageous if the TiO.sub.2
particles are coated with Al.sub.2O.sub.3 or with Al.sub.2O.sub.3
and polydimethylsiloxane.
[0046] In a preferred embodiment, the polypropylene film according
to the invention comprises at least one top ply or if necessary top
plies on both sides, composed of polymers of .alpha.-olefins having
2 to 10 carbon atoms. In general the top ply or both top plies
contain at least 70% by weight, preferably 80 to 100% by weight, in
particular 90 to 98% by weight, based in each case on each top ply,
of .alpha.-olefinic polymers described below.
[0047] Examples of such .alpha.-olefinic polymers are
[0048] a propylene homopolymer or
[0049] a copolymer of
[0050] ethylene and propylene or
[0051] ethylene and 1-butylene or
[0052] propylene and 1-butylene or
[0053] a terpolymer of
[0054] ethylene and propylene and 1-butylene or
[0055] a mixture of two or more of the stated homo-, co- and
terpolymers or
[0056] a blend of two or more of the stated homo-, co- and
terpolymers, if necessary mixed with one or more of the stated
homo-, co- and terpolymers,
[0057] in particular a propylene homopolymer or
[0058] a random ethylene/propylene copolymer having
[0059] an ethylene content of 1 to 10% by weight, preferably 2.5 to
8% by weight, or
[0060] a random propylene/1-butylene copolymer having
[0061] a butylene content of 2 to 25% by weight, preferably 4 to
20% by weight, based in each case on the total weight of the
copolymer, or
[0062] a random ethylene/propylene/1-butylene terpolymer having
[0063] an ethylene content of 1 to 10% by weight, preferably 2 to
6% by weight, and
[0064] a 1-butylene content of 2 to 20% by weight, preferably 4 to
20% by weight, based in each case on the total weight of the
terpolymer, or
[0065] a blend of an ethylene/propylene/1-butylene terpolymer and a
propylene/1-butylene copolymer
[0066] having an ethylene content of 0.1 to 7% by weight and a
propylene content of 50 to 90% by weight and a 1-butylene content
of 10 to 40% by weight, based in each case on the total weight of
the polymer blend,
[0067] being preferred.
[0068] The propylene homopolymer used in the top ply contains
predominantly (at least 90%) propylene and has a melting point of
140.degree. C. or higher, preferably 150 to 170.degree. C.,
isotactic homopolypropylene having an n-heptane-soluble fraction of
6% by weight or less, based on the isotactic homopolypropylene,
being preferred. The homopolymer has in general a melt flow index
of 1.5 g/10 min to 20 g/10 min, preferably 2.0 g/10 min to 15 g/10
min.
[0069] If required, the top ply contains the propylene homopolymer
which is described above for the base ply and whose
n-heptane-insoluble fraction is highly isotactic. The top ply
preferably essentially comprises this homopolymer.
[0070] The copolymers used in the top ply and described above have
in general a melt flow index of 1.5 to 30 g/10 min, preferably 3 to
15 g/10 min. The melting point is in the range from 120 to
140.degree. C. The terpolymers used in the top ply have a melt flow
index in the range from 1.5 to 30 g/10 min, preferably 3 to 15 g/10
min, and a melting point in the range from 120 to 140.degree. C.
The blend of copolymer and terpolymer, described above, has a melt
flow index of 5 to 9 g/10 min and a melting point of 120 to
150.degree. C. All melt flow indices stated above are measured at
230.degree. C. and under a force of 21.6N (DIN 53 735).
[0071] If required, all top ply polymers described above can be
degraded by a peroxide mechanism in the manner described above for
the base ply, in principle the same peroxides being used. The
degradation factor for the top ply polymers is in general in a
range from 3 to 15, preferably 6 to 10.
[0072] In a dull embodiment, the top ply additionally contains a
high density polyethylene (HDPE) which is mixed or blended with the
top ply polymers described above. The composition and details of
the dull top plies are described, for example, in German Patent
Application P 43 13 430.0 which is incorporated herein by
reference.
[0073] If required, the additives described above for the base ply,
such as antistatic agents, antiblocking agents, pigments,
lubricants, neutralizing agents and stabilizers, can be added to
the top ply or top plies. In a preferred embodiment, the top ply or
plies contains or contain a combination of antiblocking agent,
preferably SiO.sub.2, and lubricant, preferably
polydimethylsiloxane.
[0074] The film according to the invention comprises at least the
base ply described above and preferably at least one top ply.
Depending on its intended use, the film can have a further top ply
on the opposite side, in which case the "base ply" becomes an inner
ply. If required, an interlayer or interlayers can be applied on
one or both sides between the base ply and the top ply or
plies.
[0075] Preferred embodiments of the polypropylene film are
three-ply. The structure, thickness and composition of a second top
ply can be chosen independently of the top ply already present, and
the second top ply can likewise contain one of the polymers or
polymer mixtures which are described above but which need not be
identical to that of the first top ply. The second top ply can,
however, also contain other conventional top ply polymers.
[0076] The thickness of the top ply or plies is greater than 0.1
.mu.m and is preferably in the range from 0.3 to 3 .mu.m, in
particular 0.4 to 1.5 .mu.m, and top plies on both sides can be of
equal or different thickness.
[0077] The interlayer or interlayers can comprise the
.alpha.-olefinic polymers described for the top plies. In a
particularly preferred embodiment, the interlayer or interlayers
comprises or comprise the highly crystalline propylene homopolymer
described for the base ply. The interlayer or interlayers can
contain the conventional additives described for the individual
plies.
[0078] The thickness of the interlayer or interlayers is greater
than 0.3 .mu.m and is preferably in the range from 1.0 to 15 .mu.m,
in particular 1.5 to 10 .mu.m.
[0079] The total thickness of the polypropylene film structure
according to the invention can vary within wide limits and depends
on the intended use. It is preferably 4 to 60 .mu.m, in particular
5 to 30 .mu.m, preferably 6 to 25 .mu.m, the base ply (in the case
of laminar structures) accounting for about 40 to 100% of the total
film thickness.
[0080] The invention furthermore relates to a process for producing
a polypropylene film structure according to the invention by an
extrusion process known per se; in the case of laminar structures,
a coextrusion process, also known per se, is used.
[0081] In the coextrusion process, the ply or melts corresponding
to the ply or to the individual plies of the film is or are
coextruded through a flat die, the film thus obtained is drawn off
on one or more rollers for solidification, the film is then
biaxially stretched (oriented) and the biaxially stretched film is
thermofixed and, if required, corona-treated or flame-treated on
the surface ply intended for treatment.
[0082] The biaxial stretching (orientation) is generally carried
out successively, the successive biaxial stretching, in which
stretching is first carried out longitudinally (in the machine
direction) and then transversely (perpendicular to the machine
direction), being preferred.
[0083] Initially, the polymer or the polymer mixture of the
individual plies is compressed and liquefied in an extruder, as is
usual in the coextrusion process, and the additives added if
required can already be present in the polymer or in the polymer
mixture. The melts are then simultaneously forced through a flat
die (slot die), and the extruded composite film is drawn off on one
or more draw-off rollers, during which it cools and solidifies.
[0084] The film thus obtained is then stretched longitudinally and
transversely relative to the extrusion direction, which leads to
orientation of the molecular chains. The longitudinal stretching Us
expediently carried out with the aid of two rollers running at
different speeds corresponding to the desired stretching ratio, and
the transverse stretching is carried out with the aid of an
appropriate tenter frame. The longitudinal stretching ratios are in
the range from 5.0 to 9, preferably 5.5 to 8.5. The transverse
stretching ratios are in the range from 5.0 to 9.0.
[0085] The biaxial stretching of the film is followed by its
thermofixing (heat treatment), the film being kept for about 0.1 to
10 s at a temperature of 100 to 160.degree. C. The film is then
wound up in the usual manner by means of a winding device.
[0086] It has proved to be particularly advantageous to keep the
draw-off roller or rollers, by means of which the extruded film is
cooled and solidified, at a temperature of 10 to 100.degree. C.,
preferably 20 to 70.degree. C., by a heating and cooling
circulation.
[0087] The temperatures at which longitudinal and transverse
stretching are carried out can be varied within a relatively wide
range and depend on the desired properties of the film. In general,
longitudinal stretching is preferably carried out at 80 to
150.degree. C. and transverse stretching preferably at 120 to
170.degree. C.
[0088] After the biaxial stretching, one or both surfaces of the
film are preferably corona-treated or flame-treated by one of the
known methods. The intensity of treatment is in general in the
range from 37 to 50 mN/m, preferably 39 to 45 mN/m.
[0089] In an expedient corona treatment, the film is passed between
two conductor elements serving as electrodes, such a high voltage,
in most cases alternating voltage (about 5 to 20 kV and 5 to 30
kHz), being applied between the electrodes that spray discharges or
corona discharges can take place. Due to the spray discharge or
corona discharge, the air above the film surface is ionized and
reacts with the molecules of the film surface so that polar spots
are formed in the essentially nonpolar polymer matrix.
[0090] For a flame treatment with a polarized flame (cf. U.S. Pat.
No. 4,622,237), a direct electric voltage is applied between a
burner (negative pole) and a cooling roller. The level of the
applied voltage is between 400 and 3,000 V, preferably in the range
from 500 to 2,000 V. Owing to the applied voltage, the ionized
atoms experience increased acceleration and impinge at higher
kinetic energy on the polymer surface. The chemical bonds within
the polymer molecule are more readily broken, and the formation of
free radicals proceeds more rapidly. The thermal stress on the
polymer is in this case far less than in the standard flame
treatment, and films can be obtained in which the sealing
properties of the treated side are even better than those of the
untreated side.
[0091] The film according to the invention is distinguished by
outstanding mechanical strengths.
[0092] The modulus of elasticity of the film in the longitudinal
direction is greater than 2,500 N/mm.sup.2, preferably greater than
2,700 N/mm.sup.2, and the modulus of elasticity of the film in the
transverse direction is greater than 4,000 N/mm.sup.2, preferably
greater than 4,200 N/mm.sup.2. Surprisingly, it has been found that
no resin has to be added in order to achieve these excellent moduli
of elasticity in comparison with the prior art. According to the
prior art, about 15 to 30% by weight of resin are added to the base
ply in order to achieve the good mechanical properties. The film
according to the invention contains essentially no resin with the
result that no resin deposits occur on the screw of the extruder
and on the rollers of the longitudinal stretching unit. Moreover,
the film is distinguished by low high-temperature blocking values
and by excellent, non-blocking behavior during further
processing.
[0093] Surprisingly, even with a thickness of less than 20 .mu.m,
the films according to the invention are sufficiently rigid to
permit processing on the modern high-speed packaging machines. With
this film, it is therefore possible further to reduce the plastics
content of packaging without there being any losses in the quality
of the packaging.
[0094] The films are furthermore distinguished by a substantially
improved barrier effect, especially with respect to water vapor and
oxygen. Surprisingly, it has also been found here that, in order to
achieve these good barrier values, no resin must be added to the
film. In the case of the 25 .mu.m thick film described at the
outset in the prior art and having a water vapor transmission of
1.1 g/m.sup.2.multidot.d, the water vapor barrier effect can be
reduced, for example, to 0.9 g/m.sup.2.multidot.d without the
addition of hydrocarbon resin. In the case of films according to
the prior art, the addition of at least 5 to 10% by weight of resin
is required for this purpose.
[0095] The following methods of measurement were used for
characterizing the raw materials and the films:
[0096] Melt Flow Index
[0097] The melt flow index was measured according to DIN 53 735 at
21.6N load and 230.degree. C.
[0098] Melting Point
[0099] DSC measurement, maximum of the melting curve, heating rate
20.degree. C./min.
[0100] Water Vapor and Oxygen Transmission
[0101] The water vapor transmission is determined according to DIN
53 122 Part 2. The oxygen barrier effect is determined according to
Draft DIN 53 380 Part 3 at an atmospheric humidity of 53%.
[0102] Opacity
[0103] The opacity of the film was measured according to ASTM-D
1003-52.
[0104] Gloss
[0105] The gloss was determined according to DIN 67 530. The
reflector value was measured as an optical characteristic of the
surface of a film. Analogously to the standards ASTM-D 523-78 and
ISO 2813, the angle of incidence was set at 60.degree. or
85.degree.. At the set angle of incidence, a light beam strikes the
planar test surface and is reflected or scattered by the latter.
The light beams incident on the photoelectronic receiver are
indicated as a proportional electric value. The measured value is
dimensionless and must be quoted with the angle of incidence.
[0106] Surface Tension
[0107] The surface tension was determined by means of the so-called
ink method (DIN 53 364).
[0108] Printability
[0109] The corona-treated films were printed on 14 days after their
production (short-term evaluation) or 6 months after their
production (long-term evaluation). The ink adhesion was evaluated
by means of the self-adhesive tape test. The ink adhesion was rated
as moderate if little ink could be removed by means of
self-adhesive tape and was rated as poor if a substantial amount of
ink could be removed.
[0110] Tensile Strength, Elongation at Break
[0111] The tensile strength and the elongation at break are
determined according to DIN 53455.
[0112] Modulus of Elasticity
[0113] The modulus of elasticity is determined according to DIN 53
457 or ASTM 882.
[0114] Determination of the High-Temperature Blocking
Characteristics
[0115] To measure the high-temperature blocking characteristics,
two wooden blocks adhesively bonded to felt on one side and having
the dimensions 72 mm.times.41 mm.times.13 mm are wrapped and sealed
in the film to be measured. A weight of 200 g is placed on the
wooden blocks positioned so that the felt coverings face one
another, and this set-up is introduced into a heating oven
preheated to 70.degree. C. and is left there for 2 hours.
Thereafter, cooling is effected for 30 minutes to room temperature
(21.degree. C.), the weight is removed from the wooden blocks and
the upper block is pulled off the lower block by means of a
mechanical apparatus. The evaluation is affected over 4 individual
measurements, from which a maximum pull-of f force (measured in N)
is then determined. The specification is met if none of the
individual measurements is above 5N.
[0116] Molecular Weight Determination
[0117] The molecular weight average M.sub.w and M.sub.n (weight
average M.sub.H and number average M.sub.n) and the mean
inhomogeneity of the molecular weight were determined analogously
to DIN 55 672, Part 1, by means of gel permeation chromatography.
Instead of THF, ortho-dichlorobenzene was used as the eluant. Since
the olefinic polymers to be investigated are not soluble at room
temperature, the entire measurement is carried out at an elevated
temperature (.apprxeq.135.degree. C.).
[0118] Isotactic Content
[0119] Both the isotactic content of the homopolymer and the
isotactic content of the film can be characterized approximately by
means of the insoluble fraction of the raw material or of the film
in a suitable solvent. It has proven expedient to use n-heptane.
Usually, a Soxhlet extraction with boiling n-heptane is carried
out. In order to obtain good reproducibility, it is expedient to
fill the Soxhlet apparatus with a pellet instead of granules. The
thickness of the pellet should not exceed 500 micrometers. For the
quantitative determination of the atactic content of the
homopolymer, it is of decisive importance to ensure sufficient
extraction time. As a rule, the extraction time is in the range
from 8 to 24 hours.
[0120] The operational definition of the isotactic content
PP.sub.iso in percent is given by the ratio of the weights of the
dried n-heptane-insoluble fraction to the sample weight:
[0121] PP.sub.iso=100.times.(n-heptane-insoluble fraction/sample
weight)
[0122] An analysis of the dried n-heptane extract shows that, as a
rule, it does not comprise pure atactic propylene homo-polymer. In
the extraction, aliphatic and olefinic oligomers, in particular
isotactic oligomers, and also possible additives, such as, for
example, hydrogenated hydrocarbon resins, are also measured.
[0123] Chain Isotaxy Index
[0124] The isotactic content PP.sub.iso defined above is not
sufficient for characterizing the chain isotaxy of the homopolymer.
It proves to be useful to determine the chain isotaxy index II of
the homopolymer by means of high-resolution .sup.13C-NMR
spectroscopy, the NMR sample chosen being not the original raw
material but its n-heptane-insoluble fraction. To characterize the
isotaxy of polymer chains, .sup.13C-NMR spectroscopic triad isotaxy
index II (triads) is used in practice.
[0125] Determination of the Triad-Related Chain Isotaxy Index II
(Triads)
[0126] The chain isotaxy index II (triads) of the
n-heptane-insoluble content of the homopolymer and of the film is
determined from the .sup.13C-NMR spectrum of said homopolymer or of
said film. The intensities of triad signals which result from the
methyl groups with different local environments are compared.
[0127] With regard to the evaluation of the .sup.13C-NMR spectrum,
a distinction must be made between two cases:
[0128] A) The raw material investigated is a propylene homo-polymer
without a random C.sub.2 content.
[0129] B) The raw material investigated is a propylene homopolymer
having a low random C.sub.2 content, referred to below as
C.sub.2-C.sub.3-copolymer.
[0130] Case A
[0131] The chain isotaxy index of the homopolymer is determined
from its .sup.13C-NMR spectrum. The intensities of the signals
which result from the methyl groups with different environments are
compared. In the .sup.13C-NMR spectrum of a homopolymer,
essentially three groups of signals, so-called triads, occur.
[0132] 1. At a chemical shift of about 21 to 22 ppm, the "mm-triad"
occurs and is assigned to the methyl groups having methyl groups
directly adjacent on the left and right.
[0133] 2. At a chemical shift of about 20.2 to 21 ppm, the
"mr-triad" occurs and is assigned to the methyl groups having
methyl groups directly adjacent on the left or right.
[0134] 3. At a chemical shift of about 19.3 to 20 ppm, the
"rr-triad" occurs and is assigned to the methyl groups without
directly adjacent methyl groups.
[0135] The intensities of the signal groups assigned are determined
as the integral of the signals. The chain isotaxy index is defined
as follows: 2 ( triads ) = J mm + 0.5 J mr J mm + J mr + J rr 100
II
[0136] where J.sub.mm, J.sub.mr and J.sub.rr are the integrals of
the signal groups assigned.
[0137] Case B
[0138] FIG. 1 of the Drawing is a schematically enlarged
representation of a .sup.13C-NMR spectrum of an ethylene/propylene
copolymer. The chemical shift of the methyl groups of interest is
in the range from 19 to 22 ppm. As can be seen in FIG. 1, the
spectrum of the methyl groups can be divided into three blocks. In
these blocks, the CH.sub.3 groups appear in triad sequences, whose
assignment to the local environments is explained in detail
below:
[0139] Block 1:
[0140] CH.sub.3 groups in the PPP sequence (mm-triad) 1
[0141] Block 2:
[0142] CH.sub.3 groups in the PPP sequence (mr- or rm-triads) 2
[0143] and CH.sub.3 groups in the EPP sequence (m-chain): 3
[0144] Block 3
[0145] CH.sub.3 groups in the PPP sequence (rr-triads): 4
[0146] CH.sub.3 groups in an EPP sequence (r-chain): 5
[0147] CH.sub.3 group s in an EPE sequence: 6
[0148] In the determination of the triad-related chain isotaxy
index II (triads) of the n-heptane-insoluble content of an
ethylene/propylene copolymer, only PPP triads were considered, i.e.
only those propylene units which are present between two adjacent
propylene units (cf. also EP-B-0 115 940, page 3, lines 48 and
49).
[0149] The definition of the triad isotaxy index of an
ethylene/propylene copolymer is:
II (triads)=100.times.(J.sub.mm/J.sub.ppp)
[0150] Calculation of the chain isotaxy index of an
ethylene/propylene copolymer:
[0151] 1. J.sub.mm is given by the peak integral of block 1.
[0152] 2. Calculate the integral (J.sub.total) of all methyl group
peaks in blocks 1, 2 and 3.
[0153] 3. By simple considerations, it is possible to show that
J.sub.ppp=J.sub.total-J.sub.EPP-J.sub.EPE.
[0154] Sample Preparation and Measurement
[0155] 60 to 100 mg of polypropylene are weighed into a 10 mm NMR
tube and hexachlorobutadiene and tetrachloroethane are added in a
ratio of about 1.5:1 until a level of about 45 mm is reached. The
suspension is kept at about 140.degree. C. until (as a rule about
one hour) a homogeneous solution has formed. In order to accelerate
the dissolution process, the sample is stirred from time to time
with a glass rod.
[0156] The .sup.13C-NMR spectrum is recorded at an elevated
temperature (as a rule 365K) under standard measuring conditions
(semiquantitatively).
[0157] The following Examples illustrate the principles and the
practice of this invention without in any way limiting its
scope.
EXAMPLE 1
[0158] A transparent three-ply film having a symmetrical structure
and a total thickness of 16 .mu.m was produced by coextrusion and
subsequent stepwise orientation in the longitudinal and transverse
direction. The top plies each had a thickness of 0.6 .mu.m.
[0159] A-base ply:
[0160] 99.85% by weight of highly isotactic polypropylene from
Solvay, having the brand name Eltex P HCL 480
[0161] 0.15% by weight of N,N-bisethoxyalkylamine
[0162] The n-heptane-insoluble fraction of the film had a chain
isotaxy index of 96%, measured by means of .sup.13C-NMR
spectroscopy.
[0163] B-top plies:
[0164] 98.77% by weight of a random ethylene/propylene copolymer
having a C.sub.2 content of 4.5% by weight
[0165] 0.33% by weight of SiO.sub.2 as an antiblocking agent,
having a mean particle size of 2 .mu.m
[0166] 0.90% by weight of polydimethylsiloxane having a viscosity
of 30,000 mm.sup.2/s
[0167] The production conditions in the individual process steps
were:
1 Extrusion: Temperatures A-ply: 280.degree. C. B-plies:
280.degree. C. Temperature of the draw-off 30.degree. C. roller:
Longitudinal Temperature: 135.degree. C. stretching: Longitudinal
stretching ratio: 6.5 Transverse Temperature: 160.degree. C.
stretching: Transverse stretching ratio: 8.5 Fixing: Temperature:
110.degree. C. convergence: 20%
[0168] The transverse stretching ratio .lambda..sub.T=8.5 is an
effective value. This effective value is calculated from the final
film width W, minus twice the seam width w, divided by the width of
the longitudinally stretched film C, likewise minus twice the seam
width w.
2 Numerical data: Final film width W = 4,000 mm Final film
thickness d = 16 .mu.m Seam width w = 40 mm Width of the longitu- C
= 530 mm dinally stretched film
[0169] The film produced in this manner had the properties listed
in the Table (first line: Example 1).
EXAMPLE 2
[0170] A three-ply film having a total thickness of 16 .mu.m and
top ply thicknesses of 0.5 .mu.m each was produced as in Example 1.
The raw material composition for the base ply and for the top plies
is also the same as in Example 1. Only the conditions during
longitudinal and transverse stretching were changed:
3 Longitudinal stretching: Temperature: 135.degree. C. Longitudinal
stretching 7.5 ratio: Transverse stretching: Temperature:
160.degree. C. Transverse stretching 8.0 ratio:
[0171] The film properties are listed in the Table--second line
(Example 2).
EXAMPLE 3
[0172] The formulation for the base ply was chosen as in Example 1.
In the case of the top plies, the siloxane content was increased
from 0.9% by weight to 1.6% by weight. The slip capacity of the
film was thus considerably improved. The process conditions were
those from Example 2.
EXAMPLE 4
[0173] The formulation of Example 2 was used for the base ply. In
addition, the base ply also contained erucamide as a lubricant in a
concentration of 0.2% by weight, based on the base ply. The top
plies likewise contained the highly isotactic polypropylene from
Solvay. The silica from Example 1 was used in the same
concentration as an antiblocking agent in the top plies. The
process parameters were taken from Example 2.
EXAMPLE 5
[0174] The thickness of the base ply was retained. The top ply
thicknesses were each 1 .mu.m. The base ply corresponded to that of
Example 1. The top plies were symmetrically arranged as in Example
1 and had the following formulation:
[0175] 68.77% by weight of a random ethylene/propylene copolymer
having a C.sub.2 content of 4.6% by weight
[0176] 30.0% by weight of low density polyethylene having a melt
flow index of 2 g/10 min from Hoechst, with the brand name HOSTALEN
GD 7255
[0177] 0.33% by weight of SiO.sub.2 as an antiblocking agent,
having a mean particle size of 2 .mu.m
[0178] 0.90% by weight of polydimethylsiloxane having a viscosity
of 30,000 mm.sup.2/s
[0179] The production conditions in the individual process steps
were as stated in Example 1.
[0180] In comparison with the previous Examples, the film had a
dull appearance.
EXAMPLE 6
[0181] The formulation of the film was as in Example 1. The
stretching conditions were taken from Example 1. The film thickness
was now 20 .mu.m instead of 16 .mu.m.
Comparative Example 1
[0182] With regard to the thicknesses, the film structure and the
process conditions, there were no changes compared with Example 1.
Instead of the material from Solvay (Eltex P HCL 480) used in the
base ply, the material from Solvay, Eltex PHP 405, known from the
prior art, was now chosen. The n-heptane-insoluble fraction of the
film had a chain isotaxy index, measured by means of .sup.13C-NMR
spectroscopy, of 92%. The resulting film properties are listed in
the Table.
Comparative Example 2
[0183] Example 1 described in EP-A-0 046 833 was worked through.
The n-heptane-soluble fraction of the film had a chain isotaxy
index, measured by means of .sup.13C-NMR spectroscopy, of 93%. The
resin content in the base ply was 10% by weight. In comparison with
the film according to the invention (cf. Example 6), the barrier
values for water vapor and the tensile modulus of elasticity are
substantially lower.
[0184] As used in this application, the terms "polypropylene" and
"propylene polymer" refer to both homopolymers and copolymers
(including terpolymers, quaterpolymers, etc.) of propylene.
4 TABLE Modulus of Tensile Elongation Friction High- elasticity
strength at break Water vapor Opacity 14 days temperature DIN 53
457 DIN 53 455 DIN 53 455 trans- ASTM after blocking Film
longitudinal/ longitudinal/ longitudinal/ mission D production
Scratch character- thickness transverse transverse transverse Gloss
DIN 53 122 1003-52 B side/ resistance istics .mu.m N/mm.sup.2
N/mm.sup.2 % DIN 67 530 g/m.sup.2 .multidot. d % B' side .DELTA.
opacity N E1 16 2800/4100 200/230 140/70 100 1.4 2.0 0.23/0.25 25
1.5 E2 16 3000/4250 200/250 120/70 100 1.4 2.0 0.25/0.25 25 1.0 E3
16 3100/4300 200/270 120/70 100 1.4 2.0 0.20/0.20 25 1.5 E4 16
3300/4400 220/280 110/70 105 1.4 1.0 0.24/0.25 10 -- ES 16
2700/4050 190/240 140/70 20 1.45 40 0.20/0.20 35 0.5 E6 16
2800/4100 200/240 140/70 105 1.1 2.0 0.25/0.27 25 1.3 CE1 16
2300/4100 150/320 160/70 100 1.8 2.0 0.23/0.25 25 3.0 CE2 20
2200/4200 140/320 155/65 100 0.88 2.0 0.24/0.27 27 7.0 E = Example;
CE = Comparative Example; B side: top Layer in contact with the
chill roll; B' side; top layer which may be corona- or flame
treated if required
* * * * *